Add a way to disable some unused-ret-vals warnings via configuration

[jafingerhut]

Example:

src/instaparse/gll.clj:69:3: unused-ret-vals: Constant value is discarded: nil

Perhaps simply using the enclosing-macros approach implemented for suspicious-expression and constant-test linters will do the trick here. Overlap with #116 if so

[jafingerhut]

Part of Eastwood release 0.2.1